Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for a transportation application, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic liquid or gaseous hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon fuel, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C.
A complete fuel cell stack assembly includes fuel cell assemblies and a plurality of components known in the art as interconnects, which electrically connect the individual fuel cell assemblies in series. Typically, the interconnects include a conductive foam or weave disposed in the fuel gas and air flow spaces adjacent the anodes and cathodes of the assemblies.
A fuel cell unit may be incorporated into a modular fuel cell cassette for use in assembling a fuel cell stack. Such an assembly may comprise a metal separator plate and a metal cell-mounting plate or frame so formed that when they are joined at their perimeter edges to form a housing for the cassette, a cavity is formed between them which can contain a gas stream that feeds a fuel cell unit attached within the cassette to the mounting plate. Outboard of the fuel cell unit, the separator plate and cell-mounting plate are perforated by openings to form chimney-type manifolds for feeding fuel gas to the anode and air to the cathode, and for exhausting the corresponding gases from the stack. The fuel cell unit is attached to, and insulated from, the mounting plate. The mounting plate includes an opening through which one of the electrodes is accessible, preferably the cathode, and through which a conductive interconnect element extends to make contact with the outer surface of the next-adjacent cassette in a stack. The anode openings in the mounting plate and separator plate are separated by spacer rings such that the cassette is incompressible. The rings include openings which allow fuel gas to flow from the anode supply chimney into the anode gas channel in the cassette. For the cathode, the edges of the cathode air openings are formed similar to the perimeter of the cassette so that the edges of the respective openings in the mounting and separator plates are welded together.
In assembling a fuel cell stack from a plurality of cassettes, the mounting plate of one cassette is attached to, and insulated from, the separator plate of the next-adjacent cassette by a peripheral dielectric seal surrounding the interconnect extending from the mounting plate central opening. Thus, each cassette is at the voltage potential of the adjacent cell in a first direction by virtue of contact with its interconnect, and is insulated from the adjacent cell in the opposite direction by virtue of the peripheral dielectric seal. The cassettes are connected in electrical series and the supply and exhaust manifolds are formed inherently by the stack-assembly process.
For forming the dielectric seals between the adjacent cassettes, it is known in the prior art to use various glass and ceramic compositions based on boron, phosphate, and silica, as referenced in U.S. Pat. No. 6,430,966. These glass/ceramic sealants are also useful as dielectric insulators between adjacent cell elements at different voltage potentials. However, these sealants have some known drawbacks.
At operating temperatures, phosphate glasses are too volatile and react with the anode material to form various nickel phosphorous compounds. They also show low stability in humidified fuel gas. Borosilicates are known to react with a humidified hydrogen atmosphere to form the gaseous species B2(OH)2 at operating temperature, and thus the seal corrodes with time.
Typically, glass seals require high-temperature heat treatment (700° C.-900° C.) during manufacture of a fuel cell system, during which the glass softens and flows to fill the interface between the components and bonds to the surfaces. Upon further heating and increased time, the melt devitrifies to form the final microstructure desired for the application. This seal provides a good insulating joint and a good initial bond joint. The seal functions satisfactorily until the stack assembly undergoes multiple thermal cycles when it becomes prone to crack propagation. Because of localized differences in the coefficients of thermal expansion of the components and the glass seals, and because the glass may be progressively crystallized, the seal may fracture, resulting in gas leakage and failure of the fuel cell stack assembly. As the leak increases progressively, cell output diminishes until the total voltage output is unacceptably low.
A material used in forming dielectric sealing gaskets, known in the prior art and disclosed in U.S. Pat. No. 6,430,966, is a silicate based glass that exhibits high chemical resistance and minimal interaction with other fuel cell materials. A known material for such use is a blend of metal (M) oxides, MAOX+MBOY+SiO2, wherein MA may be barium, strontium, calcium, or a combination thereof and MB may be aluminum, boron, phosphorus, gallium or lead, or a combination thereof. MBOY modifies the softening temperature of the glass and the combination of MAOX and SiO2 offers an improved coefficient of thermal expansion. The material also provides good insulation and a good initial bond joint. However, it is prone to micro-cracking at low temperatures, increased brittleness with time during operation, and loss of bond strength with thermal cycles. Therefore, it can be difficult to maintain an adequate seal during repeated thermal cycling.
Glass seals made of compositions known in the art are also commonly prone to process variation. When fabricated from tape cast film that includes a glass frit and an organic binder, seal space can be difficult to control. This is because, during the initial heating of the SOFC stack, the binder burns out and a significant amount of shrinkage occurs. Depending on the compressive load, time under load, and heating/cooling rates in the application, it is difficult to reproduce the structure of the crystallized glass with each fabricated stack assembly. Further, flow properties of the material are very sensitive to average particle size and particle size distribution of the glass frit which is the precursor powder for the glass joint.
What is needed is a material for sealing and insulating in an SOFC system which is thermally stable over the range between shutdown and operating temperatures for both the reformer and the fuel cell assembly; which is chemically stable in oxidizing and reducing environments; which is acceptably rugged for assembly and operation of the system; which can provide a dielectric function; which matches the coefficient of thermal expansion of stainless steel elements in the fuel cell assembly; and which is compatible with other materials of the system.
It is a principal object of the present invention to hermetically seal and electrically insulate joints between adjacent cassettes in a fuel cell assembly.